(1) Technical Field
The present invention relates to high voltage power supplies for static neutralizers. More particularly, the present invention relates to high voltage power supplies that employ a resonant converter having a highly efficient circuit design. This resonant converter is suitable for driving loads with relatively high capacitance, such as emitters used in static neutralizers for generating bipolar ions by corona discharge, for enabling a compact, small foot-print implementation of these power supplies, or both.
(2) Background Art
Static neutralizers are commonly employed by the electronics industry to reduce or eliminate electro-static charge from static-sensitive components or equivalent charged objects. Static neutralizers are designed to eliminate or minimize static charge from these charged objects by generating bipolar air or, in some instances gas ions, and delivering these air or gas ions to the charged object. Static neutralizers employ a set of emitters, sometimes referred to as ionizing electrodes, corona electrodes, or corona filaments or wires. Each emitter is disposed to have a shape suitable for generating ions by corona discharge. A common emitter shape includes a long thin cylindrical shape, such as a thin wire or filament, or an end portion having a small tip radius or a sharp point. These emitters are sometimes housed in an emitter module or cell that may include one or more conducting surfaces coupled to a reference potential, such as earth ground or circuit ground. These conducting surfaces are commonly referred to collectively as a “reference electrode.”
Generating ions by corona discharge requires applying a relatively high electrical potential to at least one emitter in order to create large voltage gradients at points of high curvature on the emitter surface. When a sufficiently large voltage gradient exists on its surface, a positively charged electrode produces a cloud of positive ions by collecting electrons from nearby air molecules. Similarly, a negatively charged electrode produces a cloud of negative ions by transferring electrons onto nearby air molecules. Collectively, these positively and negatively charged ions are sometimes referred to as a bipolar ion cloud and are considered useful for static neutralization since the bipolar ion cloud contains a group of ions that have a mix of polarities that will maximize charge neutralization for a charged object selected for neutralization. The proportion of negative and positively charged ions may change depending on the environment conditions in which the static neutralizer is used.
To create a mix of ions having positive and negative charges, these static neutralizers may use an alternating high voltage waveform. Because opposite electrical charges attract, negatively charged ions are drawn to positively charged surfaces while positively charged ions are drawn to negatively charged surfaces. Once these ions reach a charged surface, the ions compensate for an excess of positive or negative charges on the surface, diminishing and thereby “neutralizing” static charge on the surface and reducing the associated hazards with these static charges.
These emitters, emitter modules or both exhibit an impedance characteristic that includes a relatively high capacitance which often exceeds 100 pF, requiring a high voltage power supply capable of driving the waveform at a frequency and amplitude suitable for creating ions by corona discharge. Since the corona voltage or waveform amplitude required to create ions by corona discharge is high, this power supply must have sufficient voltage and current driving capacity, which usually requires a large and bulky transformer. Besides the expense associated with using a large transformer, the bulk of the transformer limits placement versatility of the emitter(s), emitter module or both.
A class of circuits known variously as resonant converters or resonant inverters is commonly used to generate high voltage sine waves from low voltage DC inputs. This class of circuits is frequently used in electronic ballasts that excite Cold Cathode Fluorescent Lights and Compact Fluorescent Lights, which may respectively be referred to as “CCFL” and “CFL”. One of these topologies is the push-pull version of the current fed Class-D parallel resonant converter, sometimes called the Baxandall oscillator. An example of this architecture is shown in FIG. 1.
Using a resonant converter 8 of the type disclosed in FIG. 1 to avoid the use of a large and bulky transformer in a high voltage power supply that is suitable for use in a static neutralizer would not be obvious to try for many reasons. Resonant converters currently used in electronic ballasts that excite CCFLs normally only provide a strike voltage of approximately 2500 volts peak for a fraction of a second and then produce a sinusoidal waveform with peak amplitude that is typically less than 1000 volts during continuous operation after ignition. The amplitude of the exciting waveform typically used to create ions by corona discharge, in contrast, is higher, typically in the range of 3500 volts peak to 7500 volts peak. In addition, the capacitive load of a CCFL tube before striking is typically only around 10 pF while the capacitive load presented by an ionizer is often higher and may exceed 100 pF in some instances. The high frequency power supply driving the emitters in a static neutralizer frequently operate continuously for relatively long periods of time, while generating much higher output voltages and output currents than required of the power supplies that drive CCFLs. These factors generally make the power supplies for high frequency static neutralizers much larger, more expensive, and more difficult to design than the power supplies that drive CCFLs. Furthermore, the need to produce high frequency, high voltage waveforms in a confined space leads to a host of problems with conventional resonant converter designs as a result of spurious oscillation modes that arise from parasitic circuit elements, as well as problems concerning the reduced efficiency of compact designs and related problems associated with reliability and thermal management.
Consequently, there is a need for a new type of a high voltage power supply that is efficient yet capable of driving a high frequency, high voltage waveform onto at least one emitter so that ions are created by corona discharge. Moreover, there is a need for a high voltage power supply this not only efficient but is also low cost and suitable for compact, small foot print implementations.